Fillers in the form of finely divided particulate or plate-shaped inorganic materials are in general use in the manufacture of, for example, paper, paint, printing ink and rubber. In this connection, calcium carbonate is in common use and has the advantages of being in ample supply and cheap. However, its low acid-resistance is a disadvantage, especially in the papermaking industry. Thus, calcium carbonate, when used as a filler in paper, is inconvenient in so far as its reacts with alum which is also used in papermaking. Alum which is aluminium sulphate (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O), produces aluminium hydroxide and sulphuric acid when added to the "wet" papermaking system. The flocculation of aluminium hydroxide is capable of collecting and retaining the filler and also part of the resin-alum system for paper sizing. During manufacture, the alum flocks in the paper web are removed, and the sulphuric acid is increasingly concentrated in the white water, in spite of the buffer action of the system. The presence of calcium carbonate in a system of pH 4 will quickly consume part of the sulphuric acid, thereby forming calcium sulphate and free Ca.sup. 2+ ions. The presence of these ions in the recycled white water causes a considerable reduction of the sizing effect in the papermaking pulp, and this in turn necessitates the use of large amounts of alum to maintain stable sizing conditions.
A variety of techniques to modify calcium carbonate by surface treatment to achieve higher acid-resistance are disclosed in literature.
GB 1,295,264 discloses particulate surface-treated calcium carbonate adapted to be used as a filler and treated with an aqueous solution of, inter alia, water glass to coat the particles with a shell of, inter alia, calcium silicate, whereby it is possible to reduce solubility by at least 50% in an alum solution at a pH of about 5.5. However, the stability of this shell is not entirely satisfactory since it may break loose or be dissolved out upon further working of the paper product.
DE-A 3,014,620 discloses surface-modified calcium carbonate particles that have been treated with a solution containing specific cations of similar size, inter alia zinc ions, whereby the calcium ions in the crystal lattice are replaced by, inter alia, zinc ions, and this results in an improved acid-resistance and higher stability than with the first-mentioned technique. Nevertheless, a certain amount of alum is still precipitated in papermaking, for which reason the acid-resistance should be improved further.
SE-A 415,028 discloses the modification of inorganic fillers, for example calcium carbonate, by organosilane modification and metal silicate treatment of the filler.